Bacteriophage Hijacks Bacterial RNA-interference Mechanism

The CRISPR (clustered regularly interspaced short palindromic repeats) system is a bacterial RNA interference-like system which captures short fragments of DNA from plasmids, phage, and other nucleic acid parasites. After being expressed, these molecules then serve as a surveillance system where base pairing to homologous invading phages/plasmids subsequently leads to the digestion of the corresponding phage/plasmid. Strangely enough, this team found this anti-phage system within the genome of a phage found in Vibrio cholerae. When the authors examined the captured DNA sequences in this phage CRISPR system, they found sequences with 100% similarity to a 'genomic island' in several strains of V. cholerae. This genomic island is reminiscent of phage-inducible chromosomal islands found in other bacteria such as Staphylococcus aureus. Interestingly enough, these S. aureus genomic islands often inhibit bacteriophage production and provide their hosts with genes involved in niche expansion, such as toxin encoding genes. The V. cholerae genomic island provides its host with the ability to inhibit the growth of this particular species of Vibrio phage. However, the CRISPR system encoded by the phage enables it to grow in strains of V. cholera with this genomic island. When these sequences specific to the genomic island in the phage encoded CRISPR element were mutated, this growth advantage was removed. Interestingly, there were phage escape mutants that eventually regained the ability to grow. Upon investigating the CRISPR element of these mutants, the authors found that this mutant phage incorporated new pieces of DNA which were again, specific to the genomic island.
This marvellous example of tit-for-tat in bacteria-phage represents a new milestone in the complexity of this relationship. These results are especially interesting in light of past metagenomic research that has recovered presumable phage encoded CRISPR elements, including investigations of the human gut microbiome.